The present invention relates to active brazing solders for brazing ceramic parts of alumina, particularly of high-purity alumina.
Active brazing solders are alloys which contain at least one element having an affinity for oxygen, such as titanium. They attack the covalent or ionic bonding of the ceramic surfaces to be brazed, wet these surfaces, and thus unite chemically and mechanically with them. Therefore, brazing requires no fluxes whatsoever.
Part of these active brazing solders, if they are brittle and difficult to machine or unmachinable in the solidified condition, can be produced by melt-spinning in the form of thin strips, which can then be easily machined, e.g., stamped or cut.
Thus, shaped active brazing foil parts, such as rings, can be produced, which are placed between the ceramic parts to be brazed and are subsequently fused with the latter.
Part of the molten and solidified active brazing alloys may also be ground into powder and processed in this form into an active brazing paste, which can also be introduced between the ceramic parts, e.g., in the form of a ring, and subsequently fused with these parts.
When brazing ceramic parts of alumina, particularly of 99.9 percent, i.e., high-purity, alumina as is needed and used for capacitive or resistive ceramic pressure sensors, particularly absolute-pressure sensors, the active brazing solder must meet several requirements; in particular, it must have the following properties:
The temperature at which the sintered alumina ceramic is brazed, i.e., the brazing temperature, must be below 1000xc2x0 C., preferably between 700xc2x0 C. and 980xc2x0 C.
The brazed joint must be high-vacuum-tight over a long period of time, so that a vacuum existing during the brazing process in the chamber of a pressure sensor, for example, which is closed by the brazing, will remain unchanged.
The coefficient of thermal expansion of the active brazing alloy should be identical to that of the alumina ceramic in the entire temperature range covered during the brazing process, so that only minimal stress will be developed during cooling from the brazing temperature to the ambient temperature.
The strength of the brazed joint between the two ceramic parts must be so high that under tensile loading, fracture will result not at the joint, but in the adjacent ceramic.
The pressure resistance of the active brazing solder must be at least 2 GPa (=2 Gigapascals).
An active brazing solder which meets these requirements should also be processable into the aforementioned active brazing pastes, since the melt-spinning process, if applicable, requires costly and complicated equipment, so that the active brazing foils produced therewith are expensive.
With active brazing solders such as the zirconium-nickel-titanium alloys described in U.S. Pat. No. 5,351,938 (in the following abbreviated, as usual, as ZrNiTi alloys) not all of the above-mentioned boundary conditions can be fulfilled in a completely satisfactory manner. In particular, the above-mentioned requirement that the coefficients of thermal expansion of the active brazing solder and the alumina should be identical over the entire temperature range is not met, this requirement being based on new knowledge gained by the inventors.
It was therefore necessary, and this is the problem underlying the invention, to look for compositions of active brazing solders which are different from those of the prior art zirconium-nickel-titanium alloys.
The invention provides an active brazing solder for brazing alumina-ceramic parts which contains a maximum of 12 wt. % titanium, a maximum of 8 wt. % beryllium, and less than 16.5 wt. % iron, the remainder being zirconium and any impurities that may be present.
In one preferred embodiment of the invention, the active brazing solder contains 8.6 wt. % titanium, 4 wt. % beryllium, and 15.8 wt. % iron.
In another preferred embodiment, the active brazing solder contains 8.8 wt. % titanium, 2 wt. % beryllium, and 16.2 wt. % iron.
In a further preferred embodiment, the active brazing solder contains 8.9 wt. % titanium, 1 wt. % beryllium, and 16.3 wt. % iron.
In still another preferred embodiment of the invention, the active brazing solder contains 10 wt. % titanium and 4 wt. % beryllium, but no iron.
An essential advantage of the active brazing solders according to the invention is that they can be ground finely with a higher yield than the ZrNiTi alloys described in the above-mentioned U.S. Pat. No. 5,351,938 using equipment of comparable complexity, and that the grinding under oxygen described in U.S. Pat. No. 5,431,744 can be used.
In the oxygen atmosphere, the melted, cooled, and uncrushed pieces of the active brazing alloys of the invention begin to disintegrate into a hydride powder of the alloy (particle diameter of the order of less than 300 xcexcm) between 100xc2x0 C. and 150xc2x0 C. already at an absolute pressure of approximately 200 kPa (=200 kilopascals=2 bars). In a mill, e.g., a ball mill, this powder can be ground, under hydrogen overpressure and with little expenditure of energy, into powders with a desired mean particle size on the order of 10 xcexcm, e.g., 12 xcexcm. The hydrogen can be removed later during the brazing process.
The entire powder production process, namely hydrogenating, grinding, and screening, takes place in the absence of atmospheric oxygen. Grinding, storing, and packaging are carried out under hydrogen or inert-gas overpressure, so that air has no access. This ensures a low oxygen content in the powders, which have a high affinity for oxygen, so that one of the requirements for good brazing properties is met.